EPSC Abstracts Vol. 6, EPSC-DPS2011-1581, 2011 EPSC-DPS Joint Meeting 2011 c Author(s) 2011

Near Infrared Observations of -Like (596) Scheila

B. Yang (1) and H. Hsieh (1,2) (1) Institute for Astronomy, University of Hawaii, 2680 Woodlawn Drive, Honolulu, HI 96822 (2) Hubble Fellow

Abstract infrared, particularly in the 3µm region, provide the best chance of detecting diagnostic features (Lebofsky, Asteroid (596) Scheila was reported to exhibit a 1980; Jones et al., 1990; Rivkin et al., 2002). We per- cometary appearance and an increase in brightness on formed prompt spectroscopic observations of Scheila UT 2010 December 11. We used the IRCS spectro- using 8-m Subaru and 3-m IRTF telescopes to search graph on the 8-m Subaru telescope to obtain medium- for water ice features and to characterize the composi- resolution spectra of Scheila in the HK-band (1.4 - 2.5 tion of this asteroid. µm) and low-resolution spectra in the KL-band (2.0 - 4.2 µm) on UT 2010 December 13 and 14. In ad- 2. Figures dition, we obtained low-resolution spectroscopy us- ing the SpeX spectrograph on the 3-m IRTF telescope on UT 2011 January 04 and 05. The spectrum of Scheila shows a consistent red slope from 0.8 to 4.0 0.10 µm with no apparent absorption features, resembling 596 Scheila Spectrum spectra of D-type . An intimate mixing model suggests that the amount of water ice that might be Intimate Mixing Model 0.08 present on the surface of Scheila is no more than a few 50% AC, 49% Pyr, 1% Ice percent. The spectrum of the Tagish Lake chondrite matches the asteroid’s spectrum at shorter wavelengths 0.06 (λ < 2.5µm), but no hydration features are observed Geometric Albedo at longer wavelengths on Scheila. Our prompt obser- vations together with other studies of Scheila suggest 0.04 that its observed cometary-like activity is likely not 1.0 1.5 2.0 2.5 3.0 3.5 4.0 caused by the sublimation of water ice. Instead, the Wavelength (µm) dust and tail may be results of a recent . Figure 1: Open squares are the combination of Subaru observation and IRTF observation of Scheila from 1.0 1. Introduction to 4.0 µm. The red dashed line is the best-fit Hapke ra- diative transfer model. AC stands for amorphous car- Asteroid (596) Scheila, with a semi-major axis of bon and Pyr stands for pyroxene. No absorption fea- 2.93 AU, is located in the outer asteroid main belt, tures are observed. Our model suggests that surface where several main-belt (MBCs; Hsieh & Je- water ice is no more than 1 wt.% on this object. witt, 2006) are found. On 2010 Dec. 10.4, observa- tions by S. Larson using the 0.68 m Catalina Schmidt telescope show that Scheila exhibit a cometary appear- 3. Summary and Conclusions ance and an increase in brightness (Larson, 2010). The reported sudden activity of (596) Scheila shares sev- Our Subaru and IRTF observations of Scheila in the eral similarities with cometary outbursts. If the ob- spectral range 1.0 µm < λ < 4.0 µm are presented in served cometary activity is powered by the sublima- Figure 1, which shows that Scheila has a linear spec- tion of water ice, then icy grains may be found in trum with a reddish slope. Water ice and hydroxyl- the vicinity of the nucleus. Pioneering studies of as- bearing minerals, if present, would produce broad ab- teroids have recognized that observations in the near sorption bands in the spectral region from 2.7 to 3.4 µm (Rivkin et al., 2002). These diagnostic features, Acknowledgements however, are not observed in the spectrum of Scheila. We use an intimate mixing code to estimate an upper BY was supported by the National Aeronautics and limit to the amount of water ice on Scheila’s surface. Space Administration through the NASA Astrobi- Our model suggest that the surface of Scheila proba- ology Institute under Cooperative Agreement No. bly consists of fine-grained regolith and no more than NNA08DA77A issued through the Office of Space 1wt.% water ice. Considering all the available obser- Science. HHH was supported by the National Aero- vations, we conclude that the sudden activity of aster- nautics and Space Administration (NASA) through oid (596) Scheila is unlikely powered by sublimation Hubble Fellowship grant HF-51274.01, awarded by of water ice but is produced via an impact event. the Space Telescope Science Institute, which is oper- ated by the Association of Universities for Research in Astronomy, Inc., for NASA, under contract NAS 5- 26555.

References

Hsieh, H. H., & Jewitt, D. 2006, Science, 312, 561

Jones, T. D., Lebofsky, L. A., Lewis, J. S., & Marley, M. S. 1990, Icarus, 88, 172 Larson, S. M. 2010, International Astronomical Union Circular, 9188

Lebofsky, L. A. 1980, AJ, 85, 573 Rivkin, A. S., Howell, E. S., Vilas, F., & Lebof- sky, L. A. 2002, Asteroids III, W. F. Bottke Jr., A. Cellino, P. Paolicchi, and R. P. Binzel (eds), Univer- sity of Arizona Press, Tucson, 235